Alex Schechter

Prototype systems for rechargeable magnesium batteries.

The thermodynamic properties of magnesium make it a natural choice for use as an anode material in rechargeable batteries, because it may provide a considerably higher energy density than the commonly used lead–acid and nickel–cadmium systems. Moreover, in contrast to lead and cadmium, magnesium is inexpensive, environmentally friendly and safe to handle. But the development of Mg batteries has been hindered by two problems. First, owing to the chemical activity of Mg, only solutions that neither donate nor accept protons are suitable as electrolytes; but most of these solutions allow the growth of passivating surface films, which inhibit any electrochemical reaction. Second, the choice of cathode materials has been limited by the difficulty of intercalating Mg ions in many hosts. Following previous studies of the electrochemistry of Mg electrodes in various non-aqueous solutions, and of a variety of intercalation electrodes, we have now developed rechargeable Mg battery systems that show promise for applications. The systems comprise electrolyte solutions based on Mg organohaloaluminate salts, and MgxMo 3S4 cathodes, into which Mg ions can be intercalated reversibly, and with relatively fast kinetics. We expect that further improvements in the energy density will make these batteries a viable alternative to existing systems.

Recent studies on the correlation between surface chemistry, morphology, three-dimensional structures and performance of Li and Li-C intercalation anodes in several important electrolyte systems

Abstract This paper reviews some advances in the comparative study of lithium and graphite electrodes in a large matrix of solvents, salts and additives. The major purpose of this work was to support RD (ii) successful and useful application of AFM and EQCM in order to study the surface film formation and Li-deposition processes; (iii) understanding the correlation between the reversibility and stability of graphite electrodes in Li-intercalation processes and their surface chemistry, and (iv) finding an interesting correlation between the three-dimensional structure of graphite electrodes, the diffusion coefficient of Li + and their voltammetric behaviour in Li-intercalation processes.

New insights into the interactions between electrode materials and electrolyte solutions for advanced nonaqueous batteries

Abstract In this paper we review some recent work with Li metal and Li–graphite anodes and Li x MO y cathodes (M=transition metals such as Ni, Co, Mn). The emphasis was on the study of surface phenomena using in situ and ex situ FTIR spectroscopy, atomic force microscopy (in situ AFM), electrochemical quartz crystal microbalance (EQCM) and impedance spectroscopy (EIS). The performance of Li metal and Li–carbon anodes in secondary batteries depends on the nature of the surface films that cover them. The use of Li metal anodes requires the formation of highly uniform and elastic surface films. Thus, most of the commonly used liquid electrolyte solutions are not suitable for Li metal-based rechargeable batteries. In the case of Li–C-based batteries, the passivating films need not be elastic. Channeling the Li–C electrode surface chemistry towards the formation of Li 2 CO 3 surface films provides adequate passivation for these electrodes. This can be achieved through the use of EC-based solutions of low EC concentration (cosolvents should be less reactive than EC). An interesting finding is that the behavior of many commonly used cathodes also depends on their surface chemistry, and that their overall Li insertion processes include the step of Li ion migration through surface films. Their origin is discussed herein, as well as possible oxidation processes of the relevant solutions.

On the electrochemical behavior of magnesium electrodes in polar aprotic electrolyte solutions

Abstract Mg electrodes were studied in a variety of polar aprotic electrolyte solutions, using cyclic voltammetry (CV), impedance spectroscopy (EIS), surface sensitive FTIR spectroscopy, element analysis by dispersive X-rays (EDAX), scanning electron microscopy (SEM), and electrochemical quartz crystal microbalance (EQCM) studies. The solutions included Mg, Li, Na, K and Bu 4 N + salt solutions in acetonitrile (AN), propylene carbonate (PC) and tetrahydrofuran (THF). In addition, THF+RMgX (R=alkyl, X=Cl, Br) solutions were studied. This paper aims at providing a general description of the electrochemical behavior of Mg electrodes in different types of polar aprotic systems. It appears that Mg electrodes are spontaneously covered by surface films in most of the solutions studied. In AN and PC, solvent reduction seems to dominate surface film formation, while in THF, the solvent is inactive and, thus, reduction of salt anions such as ClO 4 − and BF 4 − leads to the precipitation of surface films. The impedance of Mg electrodes is very high, due to these surface films (several orders of magnitude higher than that of Li electrodes in the same solutions). However, the above difference in the surface chemistry is clearly reflected by the electrode’s impedance. Consequently, Mg dissolution in these solutions occurs via a breakdown of the surface films. However, it is possible to reduce the overpotential of Mg dissolution considerably by the presence of acidic species in solutions, which remove part of the surface films chemically. Reversible Mg deposition and dissolution are obtained in THF+RMgX solution due to the fact that in these solutions, irreversible formation of stable surface films on the Mg electrodes is avoided largely. However, EQCM studies showed that these processes are not just a simple two-electron transfer to Mg ions and are complicated by adsorption–desorption processes of the solution species.

The Study of Electrolyte Solutions Based on Ethylene and Diethyl Carbonates for Rechargeable Li Batteries I . Li Metal Anodes

The behavior of Li electrodes was studied in ethylene and diethyl carbonates (EC-DEC) solutions of LiAsF{sub 6}, LiClO{sub 4}, LiBF{sub 4}, and LiPF{sub 6}. The correlation of the surface chemistry to the interfacial properties, morphology, and Li cycling efficiency was investigated using surface sensitive Fourier transform infrared spectroscopy and impedance spectroscopy, scanning electron microscopy, X-ray energy dispersive microanalysis, and standard electrochemical techniques. The Li surface chemistry is initially dominated by EC reduction to an insoluble species, probably (CH{sub 2}OCO{sub 2}Li){sub 2}. Upon storage, several aging processes may take place, depending on the salt used. Their mechanisms are discussed. Although EC-DEC solutions were found to be adequate for Li ion rechargeable batteries, this work indicates that they are not suitable as electrolyte solutions for batteries with Li metal electrodes. This is mostly because Li electrodes cannot be considered stable in these systems and Li deposition is highly dendritic.

Imidazole and 1-methyl imidazole in phosphoric acid doped polybenzimidazole, electrolyte for fuel cells

Abstract Imidazole and 1-methyl imidazole (Me-Im) were used as additives in polybenzimidazole (PBI) equilibrated with phosphoric acid (PA), a system shown to be a high-temperature proton-conducting polymer electrolyte. The influence of different concentrations of this additive on the conductivity of these membranes was measured by a four-probe conductivity measurement, at temperatures in the range of 80–200 °C, under various humidity conditions. Correlation was found between the conductivity of liquid solutions of concentrated phosphoric acid and that of H 3 PO 4 in the PBI membranes.

Four-Electron Oxygen Reduction by Brominated Cobalt Corrole

The carbon-supported cobalt(III) complex of β-pyrrole-brominated 5,10,15-tris(pentafluorophenyl)corrole [Co(tpfc)Br(8)/C] is introduced as a nonplatinum alternative for electrocatalytic oxygen reduction in aqueous solutions. Through systematic work, the basic kinetic parameters of this reaction were studied, using rotating ring disk electrode electrochemical methods in the pH range of 0-11. Pronounced catalytic activity was detected in acid solutions along with shifts of the Co(II)/Co(III) and O(2) redox couples to more positive values (onset of 0.56 V at pH 0). A series of independent measurements have been used to prove that the dominant mechanism for oxygen reduction by Co(tpfc)Br(8)/C catalysis is the direct four-electron pathway to water.

Ethylmethylcarbonate, a Promising Solvent for Li‐Ion Rechargeable Batteries

Ethylmethylcarbonate (EMC) has been found to be a promising solvent for rechargeable Li-ion batteries. Graphite electrodes, which are usually sensitive to the composition of the electrolyte solution, can be successfully cycled at high reversible capacities in several Li salt solutions in this solvent (LiAsF{sub 6}, LiPF{sub 6}, etc.). These results are interesting because lithium ions cannot intercalate into graphite in diethyl carbonate solutions and cycle poorly in dimethyl carbonate solutions. To understand the high compatibility of EMC for Li-ion battery systems as compared with the other two open-chain alkyl carbonates mentioned above, the surface chemistry developed in both Li and carbon electrodes in EMC solution was studied and compared with that developed on these electrodes in other alkyl carbonate solutions. Basically, the major surface species formed on both electrodes in EMC include ROLi, ROCO{sub 2}Li, and Li{sub 2}CO{sub 3} species. The uniqueness of EMC as a battery solvent is discussed in light of these studies.

Magnesium Deposition and Dissolution Processes in Ethereal Grignard Salt Solutions Using Simultaneous EQCM‐EIS and In Situ FTIR Spectroscopy

Magnesium deposition and dissolution processes in ethereal Grignard salt solutions using tetrahydrofuran as the solvent and Grignard salt which included , , and (R = methyl, ethyl, butyl, or benzyl) are reported. dissolution and deposition in these solutions are basically reversible with an overall mass balance close to zero for complete cycles. In prolonged deposition processes, the mass accumulated per mole of electron transferred was similar to , the equivalent weight of magnesium. However, deposition‐dissolution is not a simple two‐electron process, but involves adsorption‐desorption processes of species such as and/or . These adsorption processes lead the electrodes to devel p high impedance upon storage in these solutions (up to hundreds of thousands of ). However, this passivation is not stable, and breaks down as the electrochemical processes proceed. ©2000 The Electrochemical Society

Methyl Propyl Carbonate: A Promising Single Solvent for Li‐Ion Battery Electrolytes

Methyl propyl carbonate (MPC) solutions containing Li salts can be used as a single-solvent electrolyte with addition of ethylene carbonate (EC). Graphite electrodes can be cycled at high reversible capacity in MPC solutions containing LiPF{sub 6} and LiAsF{sub 6}. The use of acyclic, unsymmetric alkyl carbonate solvents, such as ethyl methyl carbonate (EMC) and MPC in Li-ion based electrolytes, increases the stability of the graphite electrode. Whereas a small amount of EC is still needed as cosolvent in EMC solutions to obtain stable surface films on graphite electrodes, the authors show here that the surface films produced on graphite in MPC solutions (without added EC) are highly stable, allowing reversible Li-ion intercalation. To understand this trend, they investigated the surface chemistry developed on lithium and carbon electrodes in MPC solutions in conjugation with electrochemical studies.